This invention relates to the field of superplastic alloys, and particularly to a method of thermomechanical processing and superplastic forming a high-strength aluminum alloy at a higher strain rate.
Aluminum alloys containing Zn, Mg, Cu, and other elements in small quantities are highly desirable for aircraft structures because they can be heat treated to high strength (yield strength of approximately 70 KSI). These high strength alloys, as conventionally processed from cast ingots, have very large grains and they cannot be superplastically formed.
U.S. Pat. No. 4,092,181 describes a process for fabricating high strength alloys (e.g., 7075 and 7475) with a fine grain size of approximately 10 .mu.m. This four-step process utilizes static recrystallization to obtain a stable, fine-grain size prior to superplastic forming. The alloy is solution treated and overaged, and then rolled to impart high local plastic strains around the coarse, aged precipitates. During a subsequent step of static annealing, new grains are nucleated around these precipitates. However, not all of the aged particles are successful in nucleating a grain. This is due to the nonuniformity of plastic strain in the alloy matrix in the vicinity of different particles which causes high energy grain boundaries to consume lower energy grain boundaries during recrystallization. To achieve a finer grain size in such alloys, it was realized that a more uniform intense strain energy distribution in the matrix is needed.
Fine grain, high strength aluminum alloys processed according to the prior art patent can be superplastically formed into complex geometrical shapes. However, the forming rate for these alloys is rather low (approximately 2.times.10.sup.-4 s.sup.-1), requiring 70-100 minutes to form a typical part. Thus a strong need existed for achieving a finer grain alloy capable of much higher forming rates.
British Pat. Nos. 1,387,586 and 1,445,181 describe aluminum alloys which provide higher strain rates (5.times.10.sup.-3 s.sup.-1), but their yield strength is lower than that of the alloys described in the U.S. patent. The low-strength alloys contain Zr, Nb, and Ti as grain-refining agents, and they recrystallize during superplastic forming rather than during heat treatment prior to forming as described for the high-strength alloys. According to the British patents, a large amount of Zr in supersaturated solid solution is a prerequisite during casting of the alloy. During superplastic forming, the Zr precipitates develop from the supersaturated solid solution and the alloy recrystallizes to provide a grain size below 15 .mu.m. To take advantage of recrystallization during forming, the forming is done during a rapidly rising temperature, resulting in superplastic elongations of 400 to 600%.
There are many applications for the above described superplastic aluminum alloys. However, there are many structural applications which require significantly higher strength levels than these alloys or 7075 and 7475 aluminum can provide. Currently there are no high strength aluminum alloys of this type which can also be superplastically formed at a reasonably rapid forming rate. The combination of strength and forming strain rate provided by the prior art superplastic aluminum alloys is not adequate for many of the future applications.